"Melkor must be made far more powerful in original nature (cf. 'Finrod and Andreth'). The greatest power under Eru (sc. the greatest created power). (He was to make/ devise / begin; Manwë (a little less great) was to improve, carry out, complete.)
Later, he must not be able to be controlled or 'chained' by all the Valar combined. Note that in the early age of Arda he was alone able to drive the Valar out of Middle-earth into retreat."
Yet, it only took Tulkas to drive him out of Arda again; not to mention, to defeat him in a wrestling match and throw him on his face.
Of course, no 'dark lord' in JRRT's works ever was a great warrior. The way Morgoth tried to topple Fingolfin, was ludicrous and made me think of Confucius : "Do not use a cannon to kill a fly".